A new occurrence of ultrahigh-pressure metamorphism, central Macedonia, northern Greece: Evidence from graphitized diamonds?

Octahedral, tetrahedral, and cubic forms of graphite, interpreted here as p seudomorphs after diamond, have been discovered in situ in crustal metamorp hic rocks from central Macedonia, northern Greece. Several types of rocks, mainly of sedimentary origin, including eclogite, phyllite, quartzite, schi st, and amphibolite, have been identified as hosts to inferred diamonds. Al l assemblages are invariably graphitic and retrograded under greenschist-fa cies metamorphism. The graphitized diamonds themselves occur as inclusions in garnet, quartz, amphibole, and graphite, and range in size from approxim ately 2 to 300 mu m. In marked contrast with previously published Raman spe ctra of graphitized diamonds fi om crustal metamorphic rocks, the Raman spe ctra of the Greek specimens indicate very poor carbon crystallinity. This p robably resulted from a rapid phase transition induced by high contact comp ressive stress (i.e., non-hydrostatic pressure) at ultradeep shear zones an d subsequent rapid pressure release. The presence of former microdiamonds i nvalidates previous models on the geotectonic evolution of the Internal Hel lenide zones, and demarcates a new ultrahigh-pressure zone, the width of wh ich is currently uncertain, and which probably represents a Late Paleozoic suture marking the collision of individual continental blocks of unknown pr ovenance.